Manganese nitride



United States Patent poration of Delaware No Drawing. Filed May 17, 1965, Ser. No. 456,553

6 Claims. (Cl. 75-122) This invention relates to the provision of manganese nitride in a new form, and is concerned not only with the product but also with a novel mode of producing manganese nitride.

For producing a steel having free machining properties it is conventional to add a cyanarnide compound or a metal nitride to the molten steel, in the ladle, in an amount sufiicient to incorporate 0.15-0.40% nitrogen, by weight, in the steel. For such purpose, it has been proposed to add to the molten steel calcium cyanamide lumps as a source of nitrogen. However, it has been found that only a minor fraction (15-25%) of the available nitrogen of the cyanamide is recovered (dissolved) in the steel.

Another proposal involves adding to molten steel a nitrided flaky electrolytic manganese containing from about 4.5 to about 6.0%, by weight, of nitrogen as nitride. This flaky material is relatively light in weight and tends to float in the overlying slag (rather than sink through the slag and into the body of molten metal). Moreover, the flaky material is prone to ball up into a lump or mass (gobs) floating in the slag which mass only partially dissolves. Because of these disadvantageous characteristics only a small part of the nitride nitrogen of the nitrided electrolytic manganese becomes incorporated into the steel; a very substantial part being lost in the slag.

It has been proposed, also, to provide the desired nitrogen for treating molten steel in the form of briquetted nitrided ferro-chromium alloy. One such product on the market is identified as a nitrogen-containing, low-carbon ferro-chromium containing between 0.8 and 6.0% by weight of nitrogen in the form of nitrides. In order to briquet this material it is necessary to use a binder actually, sodium silicate. This commercial nitrogen-providing product, then, inevitably adds a significant amount of silicon to the steel. In situations where additional silicon in the steel makes no problem, the briquetted nitrided ferro-chromium is quite acceptable, however, in most cases a purposeful addition of silicon in a machinable steel would be intolerable. Likewise, it is technologically unacceptable to use for this purpose a nitrided metal nitride which has been briquetted by the aid of a carbonaceous binder (e.g., starch, molasses, or the like) because a purposeful addition of carbon to the steel usually is as objectionablefrom the steel users viewpoint-as is the addition of silicon. Use of sulphite waste liquor as binder would be doubly disadvantageous because the same would, or might, contribute to the steel not merely carbon but also sulphur.

It has been found that the nitrogen-providing additive must have a bulk density of at least 3.2 or 3.3 grams/ cubic centimeter in order to penetrate through the slag layer on top of the steel melt and into the melt itself. It is recognized, also, that for most applications the nitrogen-carrying additive must not measurably increase the silicon content 01- the carbon content of the steel (no binder). The additive should be readily pourable (for ease of charging) and should dissolve speedily in the molten steel. It should be provided in masses of substantial size in order that they descend into the melt promptly upon being added to the ladle contents. To be useful, the masses should contain at least 4.0% and preferably 5.0 or 5.5 wt. percent 3,389,990 Patented June 25, 1968 nitrogen (essentially all as nitride). And, of course, the additive should be sufliciently rugged to withstand considerable rough handling without serious breakage.

' It has been discovered that a highly acceptable nitrogenproviding additive is producible from a manganese-containing (e.g., manganese-rich) material, e.g., manganese metal itself or a manganese alloy such for instance as an. alloy of manganese and iron (of which alloy manganese constitutes the preponderance), by nitriding, at elevated temperature, a mass of particles of said material provided the particles have an acceptable shape and the particle size distribution of the particles in the mass is suchall minus 4 mesh, and largely within the range minus 6 and plus meshthat the mass has a bulk density of upwards from about 3.4 gr./ cc.

Since particulate manganese or manganese alloy does not occur in such particle size distribution, one step in the process of the present invention is to comminute relatively massive pieces of the manganese alloy or manganese metal by a procedure which creates a minimum of sliver particles. It has been found that comminution by ahammering form of comminuting action is preferable if not actually necessary: thus, material comminuted in a mill of the abrasion type is too slivery to be acceptable in the present process because the bulk density of a mass of such slivery particles is unacceptably low.

While in the following the invention will be described in greater particularity with reference to the production of sintered pellets or more or less rounded masses of nitrided -10 manganese-iron alloy (i.e., an alloy about 90% by weight of which is manganese and iron constitutes almost all of the remainder), it is to be appreciated that commercially pure manganese can be used as the starting material, or any carbon-free or low carbon manganese alloy containing upwards from about 80% Mn and containing not more than about 0.75 wt. percent silicon and not more than 1.8 wt. percent carbon and less than 1.0 wt. percent aluminum (preferably, free from any more than a trace of aluminum).

As just stated, one operable starting material is a 90-10 Mn-Fe alloy substantially free from aluminum and containing not more than 0.5 wt. percent carbon and not more than 0.1 wt. percent silicon. This material (coarsely crushed, if necessary) is comminuted in an impact-type mill, e.g., a hammer mill, to produce a finely divided material having maximum (or, desirably close) packing properties and, hence, a high bulk density whilst being sufficiently granular-in massed conditionto permit ready penetration thereinto and therethrough of a gas under only slight pressure. The comminution step. is so carried out that substantially all of the particles are minus 4 mesh and that the particles very largely have varying sizes within the range minus 6 plus 80 mesh size category. This grind has a desirably high bulk densitye.g., a bulk density of from at least about 3.4 to about 6.0 gm./cc'. in unpressed form.

Regarding the important feature of proper particle size distribution it is noted that in the case of commercially pure Mn or of the aforesaid 90-10 Mn-Fe alloy, the size analysis desirably should adhere to the following limits (in mesh sizes, Tyler standard) 2 V Wt. percent -6+l4 16-20 14+2O 16-20 20+40 18-22 40+-'80 16-22 80+100 3-5 l00+20O 10-15 8-12 3 In the case of one particular 90-10 alloy, optimum particle size distribution for maximum dense pack was:

W t. percent -6+l4 about 18 14+20 about 18 20+40 about 20 40+80 about 20 80+100 about 4 100+200 about l1 --200 about 9 In any and all cases, splintery particles tend to decrease the bulk density, and hence more symmetrical shapes of particles are Preferred.

The so-comminuted material, essentially dry and in the absence of any binder material, is simply poured as opposed to being compressed) into a suitable form or mold, e.g., a crucible or cup (or, even a tray) of mild steel (or, ceramic material or substantially pure iron or nickel or a nickel alloy or the like), and heated to an elevated temperature (e.g., a temperature of from about 650 to about 1120 C.) in an atmosphere of nitrogen or gas mixture rich in nitrogen (e.g., so-called cracked ammonia; or, the combustion products from burning propane or butane in air and then removing H and CO and CO) for at least 1 hour. The duration of the heatingnitrid'ing step varies with the particle size distribution, with the temperature maintained, with the tightness of pack of the particles, with the depth of the pack, with the desired nitrogen content in the final product. and other variables, but preferably is for a period of from 4 to 8 hours, at a temperature in excess of 650 C. but not higher than 1120" C. (preferably, about 940 (1.), in an atmosphere of commercially pure nitrogen at a pressure at least as high as atmospheric pressure.

During this heating-nitriding treatment the particles of manganese-rich material sinter together-without any appreciable meltinginto a rugged body which, after cooling, is capable of being very roughly handled without serious breakage. The sintered body is cooled in the furnace atmosphere. Its size is not greater than the filling of particulate material had been and usually is slightly less. Because of the very slight shrinkage the cooled body may easily be tipped out of the mold: it does not materially adhere to the metal (or ceramic material) of the mold or cup or tray in which the particulate mass had been nitrided-sintered.

The resulting product, which contains upwards from 4 wt. percent nitrogen essentially all as nitride, constitutes a very desirable nitrogen-supplying additive for the steel industry. "It is--by reason of the extensive self-bonding of particlesrugged enough to withstand repeated rough handlings; it provides a high content of nitrogen; it has a desirably high bulk density; it has a somewhat lower melting point than nitrided electrolytic Mn flakes and hence has a somewhat more rapid solution rate; it does not add to the steel any unalloyed silicon or unalloyed carbon; it preferably has a rounded form (few or no sharp edges); when charged to the ladle it promptly sinks through :the top slag and into the underlying melt. It is not a briquet (because the material is not pressed) but rather is a densely packed sinter. The bodies usually have a minimum size equivalent to one pound and may vary upward from this practical limit to as much as 10 or more pounds.

In the case of using commercially pure Mn as starting material, it is possible to combine as much as 6 wt. percent N (asn'itride) with the manganese. In the case of the above-mentioned 90-10 alloy, a nitrogen content of about 5.6 is maximum, regardless of the duration of the heating-nitriding step. In this connection it is to be noted that the commercially desired N range is from about 4.0 to about 5.5 wt. percent.

4 SPECIFIC EXAMPLE The starting material consisted of small pieces of the aforesaid 90-10 alloy of manganese and iron, which material was coarsely crushed and then comminuted in an tip-running hammer mill with 6 mesh grates. The comminuted material had the following size analysis:

This material was poured into mild steel cups of 50 milliliter capacity each, and placed 'in a tube furnace '(heated by electrical resistance type heating elements), through which nitrogen gas was passed. The furnace content was heated to about 940 C. for about 4.5 hours. The specimens were furnace cooled (in nitrogen atmosphere) and then tipped out of the cups.

The material poured into the cups had a bulk density of about 4.2 gr./cc., and the sintered product had about the same bulk density.

Analysis showed a content of nitrogen of about 5.1%, substantially all of which was in the form of manganese nitride. A drop test was performed on the sintered product; the test consisting of dropping a cup-shaped piece of the sintered product of approximately 50 cc. in volume from a height of six feet onto a steel plate of one inch thickness. The test specifies that such a piece shall not break into more than three major pieces, the total of which shall weigh at least 90% of the original piece. On the sintered material made as described in this example, it was found that a piece weighing 212 grams broke into two portions, the combined weight of which was 98% of the original piece.

Specimens so produced 'ere tested in steel production, by being added to ladle containing molten open hearth steel. They sank through the layer of slag and rapidly dissolved without floating to top and without burning in the air. Analyses of the steel before and after the addition showed that approxiamltey of the N content of the specimens was recovered in the steel.

Relative to the above specific example the following facts are significant. The starting material prior to comminution was in the form of pieces one inch and less in size. It was found that nitride formation could not be forced farther than one-eighth inch into the particles, and hence that all particles had to be not larger than onefourth inch (and preferably much smaller than onefourth inch) for satisfactorily thorough-going nitridizing. Material of this fineness is technologically unacceptable in steel making: not massive enough.

It was experimentally found that a desirable form of nitride could not be produced by effecting the nitriding of finely comminuted Mn (or, Mn alloy) in a shaft furnace or in a rotating kiln; at nitriding temperature the fine particles had a strong tendency to stick together forming unworkable chunks.

The combination of particle size (and particle size distribution) and non-slivery shape of particle constitutes the most important aspect of the present invention. One can vary the size distribution only slightly and therefrom form an indurated mass which one can crush in ones bare hand. Also, one can sub-divide the raw material in an attrition type of mill and thereby obtain particles which have the proper mesh analysis but which do not yield a proper bulk density. This is because they tend to take the form of slivers which particles do not pack sufficiently densely to give a bulk density within the range of 3.4-6.0 grams/cc. 4.2 grams/cc. seems to be the optimum bulk density, while 3.6 grams/cc. gives only a fair indurated product. In massive form, pure manganese has a bulk density of 7.44 gm./cc., while that of the aforesaid 90-10 Min-Fe alloys is 7.4 gm./ cc.

What I claim is:

1. Process of producing a nitrided sinter of a manganese-containing starting material selected from the group consisting of manganese and manganese alloys, which comprises coarsely crushing and then comminuting in an impact-type mill said starting material to the production of a comminuted material consisting of particles of nonslivery shape varying in size between 6 mesh and 325 mesh and having substantially the following size distribution:

and a bulk density of from about 3.4 to about 6.0 grams per cubic centimeter, filling a mold with the dry comminuted material, heating the material, in the mold, in an atmosphere rich in nitrogen at a temperature of from about 650 to about 1120 C. for at least one hour and until the material contains at least 1% by weight of nitrogen as manganese nitride, and until the material in the mold has become sintered, and cooling the resulting sintered body in an inert atmosphere.

2. The process defined in claim 1, in which the starting material is a manganese iron alloy containing about 10% Fe, carbon up to about 0.5%, silicon up to about 0.25%, the balance essentially all manganese.

3. The process defined in claim 1, in which the material is heated in nitrogen for a period of at least about 4 hours, and in which the nitrided material contains at least about 5.0 wt. percent nitrogen in combined form.

4. The process defined in claim 1, in which the nitriding temperature is from about 850 to about 940 C.

6 5. The process defined in claim 1, in which the particles of the comminuted material vary in size between 6 mesh and 325 mesh and in which the size analysis in wt. percent is as follows:

6. A sintered agglomerate consisting essentially of manganese and nitrogen and containing from about 3.4 to about 6.0 Weight percent nitrogen essentially all in the form of manganese nitride said agglomerate being substantially free from unalloyed silicon and unalloyed carbon, said agglomerate having a bulk density of about 3.4 to about 6.0 grams per cubic centimeter and a mechanical strength such that a cup-shaped agglomerate of approximately cc. volume dropped from a height of six feet onto a steel plate of one inch thickness does not break into more than three major pieces the total of which weighs at least 90% of the original agglomerate.

References Cited UNITED STATES PATENTS 2,860,080 11/1958 Wanamaker et al. 134.9

FOREIGN PATENTS 938,486 7/1949 Germany.

OTHER REFERENCES Steinberg, Ralph H. and Dave, Metals and Alloys, April 1944, p. 859.

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, Examiner.

W. W. STALLARD, Assistant Examiner. 

1. PROCESS OF PRODUCING A NITRIDGED SINTER OF A MANGANESE-CONTAINING STARTING MATERIAL SELECTED FROM THE GROUP CONSISTING OF MANGANESE AND MANGANESE ALLOYS, WHICH COMPRISES COARSELY CRUSHING AND THEN COMMINUTING IN AN IMPACT-TYPE MILL SAID STARTING MATERIAL TO THE PRODUCTION OF A COMMINUTED MATERIAL CONSISTING OF PARTICLES OF NONSILVERY SHAPE VARYING IN SIZE BETWEEN 6 MESH AND 325 MESH AND HAVING SUBSTANTIALLY THE FOLLOWING SIZE DISTRIBUTION: 